The primary field of application of the invention is the printing industry, in which the transmission of data to data carriers, in particular, the exposure of information carriers such as offset printing plates, lithography films or setting copies for flexographic printing processes, represents a very important process step. Since the source data used for the printing process are more and more frequently stored on a computer and then transmitted from this computer to the setting copy or a data carrier, this technology is referred to as the “Computer-to-Plate” (CtP) technology.
CtP devices known from the state of the art utilize data transmission principles, in which a flatbed or a drum is provided as the holding surface for the data carrier. The transmission head for transmitting the data and the data carrier that receives the data are always moved relative to one another in such a way that data can be transmitted over the largest portion of the data carrier surface possible.
It is known that flatbed systems operate relatively slow. Exposure systems that operate in accordance with the capstan principle transmit data with a single laser beam. In this case, each round exposure point becomes elliptical at a high deviation such that the resolution is limited depending on the distance from the central axis. Systems that operate with a X/Y coordinate table tend to form exposure strips that can only be prevented with complicated and costly machines. Although the utilization of multiple exposure heads with a series of laser diodes makes it possible to shorten the exposure times, it is very difficult to prevent this strip formation that is also referred to as “digital-optical banding.”
Interior drum systems, in which a laser beam is deviated by means of a mirror in such a way that it can sweep over the data carrier, require the utilization of particularly expensive and high-power YAG lasers, the focusing of which is relatively complicated because the lens system is situated far from the data carrier. The polygon mirror used needs to revolve with high speeds (approximately 20,000-30,000 rpm). This requires a complicated and expensive bearing arrangement for the mirror, wherein vibration-free guides and special foundations are also absolutely imperative.
An exterior drum system makes it possible to move the transmission head toward the plate surface until they are spaced apart by only a few millimeters. This allows the utilization of inexpensive laser diodes that can also be easily focused and utilized as multi-channel exposure heads. However, the installation of the data carrier on the outside of the drum is complicated with respect to constructive considerations and the handling (placing the plates into the centering pins). Damages (bending) may occur when handling thin offset plates with a thickness of 0.15 mm. In addition, the drum needs to be balanced differently-depending on the size of the plates to be exposed. When using thermal plates, dust particles are created during the exposure, wherein said dust particles are directly thrown onto and soil the lens system due to the centrifugal force.
The combined interior/exterior drum system proposed by the applicant for the present invention combines the advantages of interior and exterior drum systems and eliminates the respectively cited disadvantages. However, the bending of the plate which occurs during their installation on the drums may cause certain disadvantages to persist: offset printing plates with a thickness of 0.15 mm make it possible to produce a very intimate contact with the inside of the drum, but are very sensitive to bending such that they must be handled very carefully. Offset plates with a thickness of 0.30 mm are less sensitive to bending, but relatively stiff in the marginal region. This means that the plates do not always produce an intimate contact with the drum up to the edge. This can negatively influence the focus during the exposure, as well as the suction vacuum. In addition, the manufacture of these devices is relatively complicated and expensive. One also has to take into account that up to a third of the exposure time may be lost because the interior of the drum is open at the top in order to install the plates.